Principles and Technique of Fogging During Subjective Refraction

Anatomy and Physiology

The crystalline lens is a spherical-shaped body in the anterior part of the eye. The crystalline lens and cornea account for almost all the refractive powers of the eye.[10] The refractive index of the human crystalline lens ranges from 1.406 to 1.426.[11] The average power of the human crystalline lens is about 24.96 +/- 2.18 diopters (DS).[12]

An Asian study reported the index of power of the crystalline lens to range from 20.34 +/- 2.24 DS in a Chinese population.[13] The crystalline lens can adjust its power by a process known as accommodation. Accomodation is achieved by the actions of the zonular fibers attached to the equator of the crystalline lens. Relaxation of the zonular fibers causes the lens to assume a more spherical dimension, thereby increasing dioptric power. The opposite of this process is called disaccommodation.

Indications

The fogging process is indicated to control accommodation during retinoscopy and in the refraction of pediatric patients who have excessive accommodation.[14][15] Fogging is also indicated in the refraction of patients with asthenopic symptoms, which tend to be more significantly prevalent in hyperopes.[16][15] Fogging is appropriate when the clinician has a reasonable suspicion of a patient with an over-minus prescribed lens correction and in certain cases of ocular deviation and amblyopia.[17][18] Patients with latent hyperopia who report the same visual acuity over a range of powers will also benefit from fogging.[19]

Contraindications

Patients with very poor visual acuity may be unable to differentiate between the pre-fogging and post-fogging vision states. In patients with congenital or acquired aphakia, a condition characterized by the absence of the crystalline lens, there is no fogging effect during subjective refraction. 

The use of cycloplegic medications is common when examining pediatric patients. Cyclopegic medications are temporary acetylcholine receptor antagonists that paralyze the ciliary body rendering the eye unable to accommodate. It is improbable that fogging will affect the ocular accommodative system in an eye with medication-induced relaxed accommodation. 

The binocular balancing step of the refraction process requires that both eyes have similar visual acuity endpoints after monocular subjective refraction. Situations in which visual acuity is significantly different by one or more full lines are a contraindication for the fogging procedure and the entire binocular balancing process.

Equipment

The equipment required for the fogging process includes a trial frame with an occluder, a trial lens set or phoropter, and visual acuity charts. The Bailey-Lovey or Early Treatment Diabetic Retinopathy Study (ETDRS) visual acuity charts are preferred to eliminate the crowding phenomenon.[20]

Personnel

The fogging process may be performed by an orthoptist, optometrist, or ophthalmologist with the assistance of a refraction nurse. (Image. Optometrist Assessing Subjective Refraction)

Preparation

All necessary equipment used should be cleaned and disinfected before beginning the fogging procedure. The risk of cross-infection has moved some clinicians to develop alternative techniques to the fogging process.[21] Special attention must be paid to the lenses, as a smudged lens will cause the patient to report blurred vision, prolonging the refraction process.

Subjective refraction requires the understanding and participation of the patient. It is imperative that the patient be educated about the appearance of the fogging lenses during the fogging and subjective refraction procedures.

Technique or Treatment

The fogging procedure is performed at three points during the refraction process. During subjective refraction, fogging is performed before retinoscopy and during the initial maximum plus to maximum visual acuity (MPMVA) step. Fogging is also performed during the binocular MPMVA step.

Fogging During the Initial Maximum Plus to Maximum Visual Acuity Step

During the initial MPMVA step, fogging is performed monocularly with the contralateral eye occluded. By convention, the right eye is fogged first, followed by the left. The fogging process is preceded by inserting the objective refraction findings. This could be based on retinoscopy, autorefractometry results, or a former spectacle or contact lens prescription.

Placing sufficient plus power over the objective refraction findings produces fogging images (see Image. The Fogging Process). Historically, +0.75 DS was considered sufficient plus power at this step. However, current practice indicates this plus power may be insufficient to fog certain patients, particularly those with significant latent hyperopia. During this step, it is expected that the visual acuity should be 1 or 2 lines worse than the visual acuity obtained with the objective findings in place.

The patient should read the optotypes on the visual acuity chart stepwise from the largest to the smallest possible. If the patient is sufficiently fogged, the defogging process may begin. If the patient is insufficiently fogged, characterized by the ability to correctly read the optotypes to a level deemed too close to the pre-fogging levels,  the fogging should be increased. Repeat this process until the patient is satisfactorily fogged.  

The defogging process begins with directing the patient's attention to the largest optotype and encouraging him to read downwards as the optotypes transition from larger to smaller ones. The patient should achieve pre-fogging visual acuity. At this time, the reduction of plus spherical power or the addition of minus spherical power in 0.25 DS increments should continue stepwise until the patient reports no change in visual acuity with each incremental dioptric change. If the visual acuity chart contains a 20/20 line, the defogging process can be discontinued once the patients can correctly read all of the optotypes on that line.

Fogging During the Binocular Balancing Step

Binocular balancing tests balance the accommodation during binocular vision. The fogging process is employed in binocular balancing using the modified Humpriss and the Humphriss immediate contrast techniques. 

The distance monocular subjective findings are implemented before the modified Humpriss test is performed. The left eye is traditionally fogged with enough plus power to blur the vision by about three lines worse than the right eye. The examiner then defogs the left eye to its MPMVA. This process is repeated for the left eye with the right eye fogged.

The final prescription obtained is the subjective refraction endpoint.

Future perspectives

Artificial intelligence and machine learning programs have been implemented in different fields of medicine. Many manual techniques can be performed more quickly, accurate, and precisely using modern technology. Studies have shown that refraction results and best-corrected visual acuity determined by manual subjective refraction with fogging were similar to those obtained with algorithm-based methods, which proved significantly faster with statistically similar values.[22] Various modern instruments with automated methods are more accurate and reliable when compared to standard objective techniques in healthy individuals.[23][24] Clinical subjective refraction is needed to confirm automated results, especially in children and individuals with cataracts, astigmatism, and other ocular conditions.[25][26][27]

Complications

There are no known complications of the fogging technique. However, common errors made during the performance of the technique include under- or over-fogging and allowing too much time to pass between the removal and insertion of trial lenses.

Under-fogging occurs when the change in spherical powers is insufficient to adequately reduce visual acuity or keep accommodation in check. When under-fogging occurs, the eye continues to accommodate to bring the conoid of Sturm closer to the retina. Over-fogging occurs in the opposite circumstance. When the spherical powers are more than required, the conoid of Sturn is pushed too far anterior to the retina, and the defogging process becomes unnecessarily prolonged.

Clinical Significance

The refraction process aims to correct ametropia. The dioptric powers of the cornea and the crystalline lens predominately determine the amount of ametropia. The dioptric power of the cornea is fixed. However, the dioptric power of the crystalline lens varies with accommodation, resulting in fluctuating amounts of ametropia that can prolong the refraction process and lead to asthenopic symptoms if the ametropia is not adequately corrected.

At several points during the refraction process, the eye is exposed to different stimuli for accommodation, including the retinoscope light, the occluder, and the testing prisms. This repeated exposure necessitates the fogging carried out at the latter stages of the refraction process during binocular balancing.

Uncorrected hypermetropia or hyperopia can cause headaches, primarily when undiagnosed. The patient with uncorrected hypermetropia without corrective lenses does not complain of decreased vision.[28] Clinicians should request a complete eye examination before neurologically assessing these patients. Fogging can help determine hypermetropic conditions and proper lens prescription, attenuating symptoms in some patients.

Enhancing Healthcare Team Outcomes

The success of the subjective refraction process depends on the collaboration of the health workers attending to the patient. A patient with high blood sugar is not a good candidate for refraction. The clinical nursing staff should be trained and available to obtain vital signs or capillary blood glucose levels before refraction. The clinicians performing the subjective refraction process should obtain a comprehensive medical history before initiating the refraction process, as certain drugs and foodstuffs affect the accommodative system.[29][30] 

An interprofessional team can help achieve the best possible visual outcomes. Collaboration, shared decision-making with patients and caregivers for minors, and open communication are critical elements for good visual outcomes. Interprofessional patient care must use an integrated care pathway and an evidence-based approach. Earlier correction of hypermetropia and amblyopia results in improved patient prognoses and outcomes.